A phase change material (PCM) undergoes a phase change in response to an external stimulus, such as heat. This phase change is associated with a change in a physical property, such as electrical resistance or optical reflectivity, which can be measured to determine the phase of the material. A PCM is typically switched between a largely amorphous (high resistance) state and a largely crystalline (low resistance) state. This switching may be induced through heating the PCM by passing relatively high current through it, whereas reading or sensing the state of the PCM may be accomplished by passing relatively low current through it. More specifically, heating the high resistance, amorphous PCM to its crystallization temperature Tc for a long enough time changes the phase of the PCM to the low resistance, crystalline state (known as the SET process). Heating the PCM to an even higher temperature above the melting point Tm followed by fast quenching changes the phase of the PCM back to its amorphous state (known as the RESET process). Unfortunately, switching the PCM may subject it to significant stress, which may limit the number of useful switching cycles. Typical materials suitable for electrically switchable phase change material memory elements include the chalcogenides, such as Ge2Sb2Te5 (GST).
Memory devices incorporating a PCM are generally non-volatile and capable of high read and write speeds. Furthermore, phase change materials may be incorporated into a variety of devices and architectures, such as solid-state crosspoint arrays, in which the state of each cell is determined by measuring its resistance. In a crosspoint array, the cell size can approach approximately 4F2 (where “F” stands for “feature”, e.g., the minimum resolvable lithographic feature), which is the minimum cell size that can be effectively addressed by electrical interconnects for a two interconnect level design. In view of their high-speed performance and non-volatile nature, PCM memory devices have the potential to compete with existing memory devices. For solid state memory devices, one desired parameter of the PCM is relatively high resistivity in both the amorphous and crystalline states. High resistivities lead to a high voltage drop and higher power deposition for a given current pulse, which in turn means less current is required to switch the cell from the crystalline state to the amorphous state (and vice versa).
In addition to being suitable for use as the storage medium in solid-state memory devices, phase change materials may also serve as the storage medium in optical storage disks and scanning probe microscopy-based devices. For optical storage devices, a large change in reflectivity between the amorphous and crystalline phases is needed for optimum disk performance. Every kind of PCM-based memory device preferably includes a PCM having an amorphous phase that is highly stable against crystallization at elevated temperatures, since memory devices may need to operate at temperatures of 90-100° C. for long periods of time.
In general, the desirable properties of a PCM include high resistivity (especially in the crystalline phase to reduce the RESET current, if a solid state device is being used) and a high transition temperature to enhance thermal stability. What would be particularly advantageous for good solid-state PCM cell performance is a resistivity in the crystalline state of around 0.1 Ohm-cm, an on/off ratio larger than 100 (with this ratio being defined as the resistivity in the amorphous state to that in the crystalline state), a transition temperature above 150° C. (preferably in the 250-300° C. range for good thermal stability), and a crystallization time less than 100 ns. The present invention satisfies these requirements.